The present invention is directed to a dual path hydrostatic auxiliary drive system having hydraulic pressure, motor speed and power boundaries to limit system output.
Motor graders are large work vehicles. Typically they are provided with a main frame that is pivotally coupled to a forwardly extending frame about a vertical pivot axis. The main frame is provided with four driven wheels. Two longitudinally separated wheels are located on each side of the main frame. The main frame houses the internal combustion engine for powering the grader. The forwardly extending frame is supported on two front wheels. The forwardly extending frame is provided with a drawbar having a grader circle. A grader blade is mounted to the grader circle by a blade frame. The position of the grader blade relative to the grader frame is controlled by hydraulic cylinders. A saddle is mounted to the forwardly extending frame above the grader circle. The saddle is provided with hydraulic cylinders for manipulating the position of the grade circle relative to the forwardly extending frame.
Some graders are all wheel drive machines. Hydraulic wheel motors drive the front wheels. The motors selectively engage the front wheels by hydraulic clutches. The front wheels may be driven in an overdrive mode, where they are driven at a faster speed than the rear wheels; at an equal speed mode where they are driven at the same speed as the rear wheels; or in an underdrive mode, where they are driven slower than the rear wheels.
One manufacturer of all wheel drive motor graders uses mechanical overrunning clutches in place of hydraulically applied clutches. Another manufacturer markets an all wheel drive motor grader having a dual path hydrostatic auxiliary drive system.
It is an object of the present invention to provide an improved dual path hydrostatic auxiliary drive system having an auxiliary controller that controls the speed of the auxiliary drive wheels as a function of the speed of the main drive wheels in conjunction with various boundary conditions.
A vehicle having a frame that is propelled by main drive wheels that are driven by an engine and main transmission is provided with an auxiliary drive system for left and right auxiliary drive wheels. The left and right auxiliary drive wheels are driven by two independent hydraulic circuits that are independently controlled by an electronic auxiliary drive controller. Each of the hydraulic circuits has a variable displacement pump, a hydraulic motor and a hydraulically actuated clutch located between the motor and the respective auxiliary drive wheel. A directional control valve is located in the hydraulic circuit between the variable displacement pump and the hydraulic motor.
The auxiliary drive controller is in communications with a number of sensors. A main speed sensor provides a main speed signal indicating the speed of the main drive wheels. Auxiliary wheel speed sensors provide auxiliary wheel speed signals indicating the speed of the auxiliary drive wheels. Clutch pressure sensors provide clutch pressure signals indicating the hydraulic pressure being applied to the clutches. An engine speed sensor provides an engine speed signal indicating the speed of the engine.
The auxiliary drive controller independently controls the displacement and thereby the hydraulic output of the variable displacement pumps. The auxiliary drive controller drives the auxiliary drive wheels at a speed that is a fixed ratio to the speed of the main drive wheels. It does this by monitoring the main speed signal and comparing it to the auxiliary wheel speed signals.
A number of boundary conditions are stored in the auxiliary controller. There are three basic control boundaries stored in the auxiliary controller: a hydraulic pressure boundary, a motor speed boundary and a power limit boundary. If any of these boundaries are exceed by the auxiliary wheel speed signal, clutch pressure signal or a combination of these two signals, the auxiliary drive controller reduces the output of the respective variable displacement hydraulic pump. An engine speed boundary is also stored in the auxiliary controller. If the engine speed signal falls below this boundary the auxiliary drive system is shut down by reducing the flow of both variable displacement pumps to the lowest level possible.
A main clutch is drivingly positioned between the main transmission and the engine for modulating the speed of the main transmission. A clutch activation switch signals the main transmission controller and the auxiliary drive controller when the clutch is engaged and disengaged. The auxiliary controller is provided with a clutch mode selector switch having first and second modes of operation. In its first, OFF, position the auxiliary drive system shuts down by reducing the output of the variable displacement pumps to the lowest level possible whenever the main clutch is engaged. In its second, ON, position the auxiliary drive controller attempts to mirror the operation of the main clutch in controlling the output of the variable displacement pumps. The auxiliary drive controller uses the engine speed signal and the main speed signal to calculate the slip between the engine and main transmission when mirroring the operation of the main clutch.